This invention relates to a method of controlling the air-fuel ratio of a mixture being supplied to internal combustion engines, and more particularly to a method of this kind which is able to enhance the conversion efficienty of an exhaust gas-purifying device arranged in the engine exhaust system and hence improve the exhaust emission characteristics of the engine.
Internal combustion engines in general are equipped with exhaust gas-purifying devices arranged in the engine exhaust system to reduce the amounts of toxic components in exhaust gases emitted from the engine to thereby improve the exhaust emission characteristics of the engine. As such exhaust as-purifying devices there are generally employed three-way catalytic converters which operate to reduce all CO, HC, and NOx in the exhaust gases. In order to obtain the best conversion efficiency of such three-way catalytic converters, the air-fuel ratio of a mixture being supplied to the engine is controlled to a stoichiometric ratio in response to output from an exhaust gas sensor which is arranged in the engine exhaust system to detect the concentration of a specific component, e.g. oxygen, in the exhaust gases.
Conventional three-way catalytic converters have the tendency to achieve higher rates of reduction of CO and HC when the air-fuel ratio is leaner than the stoichiometric ratio, and a higher rate of reduction of NOx when the air-fuel ratio is richer than the stoichiometric ratio, respectively. Therefore, in order to obtain the best conversion efficiency, the air-fuel ratio has to be controlled to the stoichiometric ratio that is a value of compromise at which all the rates of reduction of CO, HC and NOx can be sufficient.
As one of conventional air-fuel ratio feedback control methods, a method is known e.g. from Japanese Provisional Patent Publication (Kokai) No. 61-272432 assigned to the assignee of the present application, which comprises comparing an output value indicative of sensed oxygen concentration in engine exhaust gases from an oxygen sensor arranged in the engine exhaust system with a predetermined reference value, and effecting feedback control of the air-fuel ratio to a target value (e.g. stoichiometric ratio) in such a manner that proportional control is applied by increasing or decreasing the air-fuel ratio by a first correction value whenever the output value from the oxygen sensor changes from a rich side to a lean side or vice versa with respect to the predetermined reference value, and integral control is applied by increasing or decreasing the air-fuel ratio by a second correction value upon the lapse of a predetermined period of time so long as the sensor output value remains on the rich side or on the lean side with respect to the predetermined reference value.
However, the above known method still remains to be improved if applied when the vehicle equipped with the engine is cruising at a high speed. That is, according to the known method, the first correction value is set to a smaller value at the time of correcting the air-fuel ratio to the rich side, as compared with the time of correcting the air-fuel ratio to the lean side, when the engine is in a high load condition such as engine acceleration, to thereby prevent increase in the amounts of CO and HC in the exhaust gases as caused by enriching of the mixture when the engine is in the high load condition. As a result, when the vehicle is cruising at a high speed where the engine load is high, the first correction value is thus set to a smaller value by which the air-fuel ratio is corrected toward the rich side. This, however, results in the air-fuel ratio becoming lean, making it difficult to reduce the amount of NOx to a sufficient degree.